This invention relates to a papermaking machine including a drying apparatus for drying a web of paper emerging from its press section. More particularly, this invention relates to an apparatus for drying a web in which the web is transferred between dryer groups, thereby permitting threading of the web without the assistance of threading ropes and the drying of both sides of the web while restraining the web by and against felts.
The classic paper dryer consists entirely of two or more double-felted double tier dryer groups. (Each dryer group is the group of dryer cylinders of a single-felted dryer engaging a single felt or the group of dryer cylinders of a double-felted dryer group engaging a pair of upper and lower dryer felts; a typical dryer or dryer section has several dryer groups.) A series of two double-felted double tier dryer groups is shown in U.S. Pat. No. 3,263,344, issued to Stickler in 1966, in FIG. 2. In each double-felted, double-tier dryer group of Stickler, there are two horizontal rows or "tiers" of dryers, one tier (dryers 16a and 16b) above the other tier (dryers 13a, 13b, and 13c). The web is held against one dryer (13a) of the lower tier, then is transferred to a dryer (16a) of the upper tier, then progresses to the next dryer (13b) of the lower tier, then goes to the next dryer (16b) of the upper tier, and so forth.
This type of dryer group is called a "double-felted" group because it has two felts. The upper felt wraps around the upper part of each dryer in the upper tier, and the lower felt wraps around the lower part of each dryer in the lower tier. When the web is between the top felt and a top dryer, the bottom of the web is against the dryer. When the web is between the bottom felt and a bottom dryer, the top side of the web is against the dryer. The web is thus alternately heated on its top and bottom sides as it passes over dryers of the lower and upper tiers.
As the web leaves a lower dryer to go to the next upper dryer, the felt and the lower dryer surface separate so the web can transfer to the top dryer. As the web is led away from both the lower dryer and the lower felt, it is not touching anything on either side as it follows a long path from one dryer to the other. Such an unsupported length of the web is called an open draw. When the web reaches the next upper dryer, the upper felt and the dryer surface come together, with the web between them, to bring the web into contact with the next upper dryer. Essentially the same procedure is followed to transfer the web from an upper dryer to a lower dryer.
In a double-felted double tier dryer, each transfer from one dryer to the next within a dryer group, and/or from one dryer group to the next, has required the introduction of an extended open draw, typically more than several feet (about a meter) long and at least about 16 inches (over 400 mm) long.
The faster a paper machine can be run without creating runability problems, the more efficient the machine becomes. ("Runability" is conventionally defined as the degree to which a web can be established and maintained on the machine without breaking. One measure of runability is the frequency of web breaks.) When the speed of a conventional papermaking machine is increased, however, a serious flutter problem develops in the dryer: the paper web or sheet flutters undesirably in some or all of its open draws as the sheet progresses through the double-felted double tier dryer group, or from one such group to another.
While all straight, unsupported runs of a web or felt flutter to some degree, such flutter becomes undesirable when it has a high amplitude and low frequency, particularly in a long open draw, and particularly when the web is still quite wet. High amplitude, low frequency flutter causes the web to fold, crease, or break. Web flutter is speed dependent, and undesirable flutter occurs or increases at higher web speeds.
The web flutters as it passes through a long open draw because it is rapidly moving without support through the surrounding air and the surrounding air is turbulent. The amount of flutter depends on the length of the draw, the width of the paper web in the machine, the speed of the web passing through the machine, the basis weight of the web, the machine design, the permeability of the felt, and many other factors.
Flutter of a paper web can be compared to the snapping of a flag which is mounted on the antenna of a rapidly moving automobile, or which is exposed to a high wind as it flies on a flagpole. In the case of a modern paper machine, the web itself is moving at a speed greater than half a mile a minute in some instances.
A fluttering web is subject to frequent web breaks, which are expensive and time-consuming to correct. Even an occasional web break is a very big problem. It can damage the felt and even the machine, and it inevitably causes production of paper to stop until any necessary repairs can be made and the web can be re-threaded in the running machine.
While a web break is being corrected, a web of undried paper as wide as the machine (often about 30 feet or nine meters wide) and miles (several km.) long is formed and must be collected, broken up, mixed with a much larger quantity of water, and recycled in the paper machine. Machine speeds, and thus the amount of paper a machine could produce, were limited prior to the present invention by the need to avoid an excessive number of web breaks by keeping the web speed low enough to minimize its flutter in open draws. Even after taking this precaution, web breaks were a common occurrence.
An early attempt to minimize undesirable sheet flutter has been the use of a single-felted double tier or serpentine dryer group in place of one or more double-felted double tier groups. In a serpentine dryer group, the dryer cylinders are again arranged in two tiers, and the web path is the same, but the web and a single felt follow the same path between respective top and bottom cylinders. The serpentine configuration is illustrated by FIG. 2 of U.S. Pat. No. 4,202,113.
The serpentine configuration, although reducing the problem of undesirable sheet flutter, introduces several disadvantages. First, the heat transfer from the bottom dryer cylinders is substantially reduced because the wet web is not in direct contact with the bottom cylinders. The felt is interposed between the web and the drying surfaces of the bottom cylinders. Second, the web has a tendency to sag or otherwise separate from the felt because the web travels outside the felt as it wraps around the bottom cylinder. The web can thus be influenced by the moving air and gravity, and can separate from the felt to form a bubble or flutter. Third, the initial threading of the web is not particularly easy.
U.S. Pat. No. 4,359,827, issued to Thomas on Nov. 23, 1982, represents an attempt to solve the problems of the single-felted double tier dryer by, among other things, providing a comprehensive series of vacuum boxes and grooved dryer cylinders everywhere along the web run where the web is not captured between a felt and a dryer. The vacuum boxes and grooved rolls are intended to exert a normal force keeping the web on the felt at all points, both within a dryer group and in the transfers between dryer groups. The vacuum boxes, which are sometimes referred to herein as "restraining devices," greatly complicate the design of the dryer, prevent the removal of broke, require an extremely high vacuum system capacity, and tend to wear the felts out quickly. The Thomas design thus has never been used in a commercial machine.
In the BelRun top-felted series of single tier dryer groups sold by Beloit Corporation, the problems of the serpentine double tier design have been addressed to a large degree. BelRun is a registered trademark of Beloit Corporation. Single tier dryer groups may be top-felted (meaning that the bottom surface of the web contacts each dryer cylinder and the felt runs over the top of each dryer cylinder) or, less commonly, bottom-felted (meaning that the top surface of the web contacts each dryer cylinder and the felt runs under the bottom of the dryer cylinder).
In a BelRun group, particularly at the wet end of the dryer, the bottom, ineffective dryers of the serpentine double-tier machine are replaced by vacuum rolls disposed below and between each pair of the dryer cylinders of a top-felted single tier of dryer cylinders. The vacuum rolls are in close proximity to the adjacent dryer cylinders, and a felted run of the web passes from the preceding drying cylinder to the vacuum roll, and then to the next dryer cylinder.
The felted runs of a BelRun series of dryer groups are kept short to prevent the web from departing from the felt, and thus to eliminate or at least minimize flutter, when the web and felt pass from one roll to the next. Each vacuum roll draws the web against the felt as the felt traverses the vacuum roll to restrain the web against the felt. The felt tension directly holds the felt against the dryer cylinder. The single tier dryer group thus positively keeps the web and felt together as it conveys the web through the group.
Several top-felted single tier groups can be arranged in a series, with lick-down transfers between each group, so there is no open draw in the group to group transfer. A BelRun dryer section consisting of three single tier groups united by lick-down transfers is illustrated by FIG. 10 of Linderot, "Zehn Jahre Erfahrung mit Geschlossener Bahnfuhrung in der Trockenpartie," Wochenblatt Fur Papierfabrikation, August, 1986, page 623 at 628.
Recent installations of this type of dryer group have shown that the single tier concept can be extended to include a large number of dryers without any adverse effect on web runability. A single tier dryer section has good runability because the vacuum rolls are capable of conveying the web along the felt-supported spans and a lick-down transfer can transfer the web from one felt to the next without the need for sheet tension. In the prior art, sheet tension commonly is created by providing a substantial positive speed difference or "draw" between adjacent drying groups, typically a difference exceeding 10 feet (about 3 m.) per minute.
Each top-felted single tier dryer group in a BelRun dryer section principally dries the same side (the bottom) of the web. If the web is dried from one side only, the resulting dried paper may curl. "Curl", according to "Pulp and Paper Dictionary" by John R. Lavigne, published 1986, is "a paper or paper board deformation caused by non-uniform distribution of strains and stresses throughout the sheet as a result of uneven internal moisture and conditioning." Curl can be minimized or eliminated by drying the web from both sides, but two-sided drying requires a transfer point in which the web is transferred from the felt of a top-felted dryer or dryer group directly to another felt of a bottom-felted dryer or dryer group. This felt-to-felt transfer introduces substantial open draws unless further measures are taken to eliminate them.
An object of the present invention is to overcome the aforementioned inadequacies of the prior art apparatus and to provide a drying apparatus which contributes a significant and non-obvious contribution to the paper drying art.
Another object of the present invention is the provision of an apparatus for drying a web of paper which completely eliminates the need of extended open draws, at least in the wet end of the dryer section.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which blow boxes and other restraining devices adjacent to the felted runs of the dryer section are unnecessary.
Another object of the present invention is the provision of an apparatus for drying a web of paper which permits automatic threading of a tail and subsequent web.
Another object of the present invention is the provision of an apparatus for drying a web in which open access to the dryers and vacuum rolls is provided.
Another object of the present invention is the provision of an apparatus for drying webs ranging from lightweight grades to heavy board.
Another object of the present invention is the provision of an apparatus for drying a web of paper in which the felt-supported draws are very short.
Another object of the present invention is the provision of an apparatus for drying a web of paper at a high speed substantially without flutter along the felted draws.
One or more of these objects, as well as other objects, features, and advantages of the present invention which will be apparent to those skilled in the art, are met by the present invention.